Pteridine coenzymes are those pteridine derivatives which have been reported to serve as growth factors for Crithidia fasciculata, coenzymes in hydroxylation reactions, and cofactors in still other unknown reactions. The information on the structure of these pteridine coenzymes indicates a pterin derivative (2-amino-4-hydroxypteridine) with a three carbon substituent on position 6. The pteridines associated with the pteridine coenzymes are biopterin and L- threoneopterin. The compounds which have been reported to occur in mammals, amphibia, insects, protozoa and bacteria originate from a guanine compound. Accordingly, the synthesis and regulation of biopterin is being studied, using enzyme preparation from organs of Syrian golden hamsters, chickens and other animals. Experiments will be carried out using radioactive precursors. Conventional methods will be used to purify the enzyme preparations; radioactive techniques; spectrophotometry and chromatography. A new procedure using high pressure liquid chromatography will be developed for pteridine analyses.